The present invention relates to a method for controlling a motor-driven compressor configured to be installed in a vehicle.
Conventionally, there is a motor-driven compressor that is configured to be installed in a vehicle and is driven by an inverter. The inverter receives electric power from a battery of a vehicle and converts DC power into AC power. The motor-driven compressor is driven at a number of revolutions commanded by a control device installed in the vehicle. The number of revolutions commanded by the control device is represented by discrete values. Therefore, if the number of revolutions is changed frequently in response to commands by the control device, rapid increase or decrease of the number of revolutions repeatedly happens, thus aggravating noise and vibration (NV). To solve this problem, as shown in FIG. 4, the motor-driven compressor is driven at a number of revolutions having a change per unit time that is lower than that of a commanded number of revolutions, which is commanded by the control device.
The motor-driven compressor receives electric power from the battery of the vehicle and is influenced by the battery voltage. For example, if the motor-driven compressor operates at a high number of revolutions and is stopped when the battery voltage is low, the voltage of the counter electromotive force (the counter electromotive voltage) of the motor of the motor-driven compressor may exceed the battery voltage. If this is the case, an electric current flows from the motor to the battery via the inverter of the motor-driven compressor. At this time, if the counter electromotive voltage of the motor is high, an excessive electric current flows through the inverter, thus damaging the switching elements of the inverter. To avoid this, the maximum number of revolutions at which the motor-driven compressor can be operated must be limited based on the battery voltage.
Japanese Laid-Open Patent Publication No. 7-120081 discloses an inexpensive and small-sized control-drive device for a motor-driven compressor that prevents a motor-driven compressor from being stopped through overcurrent protection effectuated by voltage change of a power source and, without adversely influencing lubricating function in the compressor, decreases the blown air temperature at a transient time of air conditioning and increases the increase speed of the blown air temperature. The control-drive device includes a means for generating DC voltage, a motor-driven compressor that incorporates a motor and compresses refrigerant for air conditioning, and an inverter. The control-drive device causes the increase speed of the frequency or the increase speed of the voltage of the inverter to be higher in a low number-of-revolutions range in a variable number-of-revolutions range than in a high number-of-revolutions range. The control-drive device also causes the decrease speed of the frequency or the decrease speed of the voltage of the inverter to be higher in the high number-of-revolutions range of the variable number-of-revolutions range than in the low number-of-revolutions range.
The battery voltage varies depending on the condition of the vehicle. Thus, the power source voltage of the motor-driven compressor can rapidly decrease in some cases. At the time of such rapid decrease of the power source voltage, even if a rapid decrease of the number of revolutions of the motor-driven compressor is attempted, the number of revolutions decreases only slowly at a decrease rate that has been determined in consideration of the NV. Therefore, if the motor-driven compressor operates at a high number of revolutions and is stopped when the battery voltage is low, the amount of the electric current flowing in the motor of the motor-driven compressor is greater than before the compressor is stopped as shown in FIG. 5. An excessive electric current may thus flow through the battery, damaging the inverter.
To prevent damage to the switching elements of the inverter caused by an excessive electric current flowing through the inverter, switching elements having a high withstand voltage may be employed. However, this increases costs and enlarges the inverter.
According to Japanese Laid-Open Patent Publication No. 7-120081, the decrease speed of the frequency or voltage of the inverter is higher in the high number-of-revolutions range of the variable number-of-revolutions range than in the low number-of-revolutions range. However, the publication does not include description regarding damage to the switching elements of the inverter caused by stopping the motor-driven compressor operating at a high number of revolutions when the battery voltage is low.